Related ad hoc networks have been used as a network in which communication nodes wirelessly communicate with each other directly without any access point. Such an ad hoc network has two major routing technologies to establish a route for data transfer between nodes: proactive routing and reactive routing. With the proactive routing technology, each node broadcasts a hello frame to neighboring nodes, thereby periodically collecting the route cost from the nodes on the network to update the route to be used for data transfer to an optimal route. With the reactive routing technology, a node serving as a data source searches an optimal route immediately before routing, thereby establishing the communication route.
In particular, with the reactive routing technology, a node on the network broadcasts a frame called a route request (RREQ) frame to the peripheral node(s) to discover an optimal route. If the RREQ reaches an intended node (e.g., a gateway node), the node creates a route reply (RREP) frame and returns (unicasts) the RREP along the route the RREQ has passed through to the node serving as the source of the RREQ. This establishes a two-way communication route between the above-described node as the source and the intended node. After that, data is transmitted and received by being transferred between the nodes along the established communication route.
FIG. 11 is a diagram for explaining a frame transfer control method in a related ad hoc network 100. As illustrated in FIG. 11, the node 102 receives an RREQ from the source node 101 through two routes. A first route is used by the node 102 to directly receive an RREQ by broadcast from the node 101. A second route is used by the node 102 to receive an RREQ by broadcast from the node 101 through the node 103. This enables the node 102 to receive two identical RREQ frames transmitted through two respective routes, which is likely to cause congestion of the network if both frames are transferred by broadcast to the subsequent node 104. To avoid the congestion, the node 102 broadcasts the firstly received RREQ to the next node 104 and then discards the secondly received RREQ that has passed through the node 103. The order of the frame to reach a node usually depends on the number of nodes the frame passes through (the number of hops); therefore the RREQ that has passed through the node 103 is discarded at the node 102.
Usually, however, a shorter distance between nodes through which the frame passes, increases the quality value of a frame. The frame that passes through the route R101 with a small number of hops and a long distance between the nodes is, therefore, more likely to be lost due to a packet loss, than the frame that passes through the route R102 with a large number of hops and a short distance between the nodes. If the node 102 discards the RREQ frames except for the firstly received RREQ frame, the subsequent high-quality RREQ frames that reach the node 102 later are discarded. As a result, a low-quality RREQ frame is transferred by broadcast to the node 104. As described above, the route to the destination node of the RREQ is established by returning the RREP along the route through which the RREQ is transmitted. This leads to inclusion of a low-quality transfer route in the route for data transmission and reception, thereby disturbing the optimal routing.
The ad hoc network 100 may broadcast both of the two identical RREQ frames transmitted through the two routes at the node 102 for the purpose of improving the communication quality of the route. Specifically, the node 102 maintains the frame ID and the quality information of the received RREQ associated with each other. If the node 102 receives a new RREQ with higher quality than the previously received RREQ, the node 102 also broadcasts the newly received RREQ. Subsequently, the node 104 that is the next node to the node 102 selects the best quality RREQ out of the received RREQs. The node 104 broadcasts the selected RREQ to the peripheral node(s).
Related technologies are described in Japanese Patent No. 4023681, Japanese Laid-open Patent Publication No. 2011-239341, and Japanese Laid-open Patent Publication No. 2010-239248, for example.
With the above-described related frame transfer control method, the node (e.g., the node 102 in FIG. 11) broadcasts an undesirable RREQ that is not used at the peripheral nodes (e.g., the node 104 in FIG. 11). In particular, if a node is on the way of a route, that is, the node is apart from the source node of the RREQ with a predetermined number of hops interposed therebetween, the node receives a plurality of RREQs broadcasted by the nodes through a plurality of routes, respectively. This leads to congestion of the network and the resulting collision of frames.
With the above-described related frame transfer control method, the node receives RREQs that have passed through a plurality of routes. Receiving an impractical RREQ with low quality may advance a timeout of receiving a subsequent RREQ. In other words, the communication with a small number of hops may inhibit the communication with an appropriate number of hops due to the timeout, thereby deteriorating the communication quality of the network. FIG. 12A is a diagram of an example of a frame transfer route in a related ad hoc network 200. As illustrated in FIG. 12A, the number of hops of the RREQ frame transfer route R201 is “2”, which is smaller than both the number of hops of the RREQ frame transfer routes R202 “5” and the number of hops of the RREQ frame transfer routes R203 “6”. As a result, the RREQ that passes through the frame transfer route R201 reaches the node 203 earlier than other RREQs. This deteriorates the communication quality of the network because the frame transfer route R201 has a longer distance between the nodes.
FIG. 12B is a diagram for explaining a transfer timeout in the related ad hoc network 200. As illustrated in FIG. 12B, in the ad hoc network 200, the RREQ that has passed through the transfer route R201 with a small number of hops reaches the node 203 first, so the time of receiving the RREQ serves as the starting point of the wait time T1 for a new RREQ. When the RREQ that has passed through the transfer routes R202 and R203 with a large number of hops reaches the node 203, the wait time T1 has already elapsed. There is a concern therefore, that the RREQ received through the transfer routes R202 and R203 is not broadcasted due to the timeout. In this example, only the low-quality RREQ is broadcasted at the timing t1 within the above-described wait time T1 at the node 203. This may disable the high-quality RREQ received through the transfer routes R202 and R203 to reach the neighboring node 213, thereby deteriorating the communication quality of the established route.